In machining with a laser beam, a work such as a metal plate is instantly and partially melted, whereby it is possible to execute machining for minute dimensions with higher precision as compared to the press machining and etching machining based on the conventional technology.
In order to minutely machine a work with high precision in a machining method using a laser beam, it is required to finely converge a laser beam through a converging lens, and in actual machining it is essential to converge the laser beam at a point on the work through the converging lens so that a converging point, which is a focus position, is not displaced. The reason for this is that it becomes impossible to execute minute machining with high precision or it becomes impossible to execute machining itself if the focus position is displaced.
A converging position maintaining apparatus based on the conventional technology for preventing a converging position from being displaced uses, as shown in FIG. 11, a contact-type displacement meter for controlling a distance between the converging lens and the work at a constant value.
In FIG. 13, a reference numeral 1 shows a laser machining head, and a nozzle 2 is provided at an end (lower end) of the laser machining head 1.
The laser machining head 1 is cylindrical and has a converging lens 3 therein, and a laser beam L is given into the cylinder by a laser oscillator not shown herein. The laser beam L given to the laser machining head 1 is converged on a top surface of a work W by the converging lens 3 and irradiated onto the work W by the nozzle 2. An assist gas inlet opening 4 is provided in the nozzle 2.
The laser machining head 1 moves in Z axial direction (in the vertical or focusing direction in FIG. 13) by a Z-axial servo motor (not shown in the drawing) driven and controlled by an NC device 20.
A contact-type displacement meter 5 is removably set in the laser machining head 1. The contact-type displacement meter 5 comprises an external case 6 integrated with the laser machining head 1, a movable shaft 7 which is supported by the external case 6 and moves upward and downward, a contact 8 having a form like a plate piece which is fixed at the lower end of the movable shaft 7 and contacts the work W, a shaft driving section 9 for driving the movable shaft 7 upward and downward for pushing the contact 8 against the work W, and a position detecting section 10 for detecting a displacement (position) of the movable shaft 7 in the vertical direction.
The work W is mounted on frog pins 12 provided on a driver base 11 and supported by the frog pins 12 at many points and also at a specified height position.
The driver base 11 is moved in the X-axial direction (leftward and rightward directions in FIG. 13) and Y-axial direction (a direction perpendicular to the plane shown in FIG. 11) by X-axial and Y-axial servo motors (not shown in the drawing) driven and controlled by the NC device 20.
Operations in the conventional method based on the configuration as described above is explained below.
The contact-type displacement meter 5 is removed from the laser machining head 1, the laser machining head 1 is positioned in the Z-axial direction so that the focus position of the laser beam L is positioned on the work W, and a distance between the nozzle 2 and the work W is adjusted.
In this state, the contact-type displacement meter 5 is placed to the laser machining head 1, the contact 8 is contacted with the work W, and a position signal (a signal showing displacement of the movable shaft 7) output from the position detecting section 10 at this point of time is stored as a focus position signal in the NC device 20.
As shown in FIG. 13, when the work W is inclined, the contact 8 moves upward or downward corresponding to a difference in height due to the inclination; this is detected by the position detecting section 10; and the NC device 20 moves the laser machining head 1 (the converging lens 3) upward or downward with the z-axial servo motor (not shown in the drawing) so that positional deviation between a position instructed by a position signal output from the position detecting section 10 and a position instructed by the focus position signal is not generated, in other words, so that the focused state is maintained.
Because of the operations described above, the focus position is maintained at a contact position against the work W even when the work W is inclined.
One of preparatory operations of laser beam machining is focusing of a laser beam. As a conventional type of focusing apparatus for focusing a laser beam, the apparatus shown in Japanese Patent Publication No. SHO 60-166185 is known. This focusing apparatus comprises, as shown in FIG. 15, a beam splitter 30 provided at an inclination of 45 degrees in a transmission path of the laser beam L, a first laser intensity sensor 31 and a second laser intensity sensor 32 provided at both sides of the beam splitter 30, a divider 33, and a meter 34.
In this focusing apparatus, a portion of a laser incoming to the work W is guided to the second laser intensity sensor 32 because of a spectral function by the beam splitter 30 and at the same time a portion of light reflected from the work W is guided to the first laser intensity sensor 31; intensity of the laser incoming to the work W is detected by the second laser intensity sensor 32; intensity of the reflected beam (reflected laser beam) from the work W is detected by the first laser intensity sensor 31; a ratio c=a/b wherein a is an output from the first laser intensity sensor 31 and b is an output from the second laser intensity sensor 32 is computed by the divider 33; an output (a ratio c) from the divider 33 is outputted to the meter 34 as a numeric value indicating a focusing degree, and the numeric value indicating a focusing degree is displayed by the meter 34 so that the value can be visually observed quantitively.
Because of this, it becomes possible to execute focusing for seeking a focal point where intensity of the reflected beam from the work W reaches the maximum by irradiating the laser beam L onto the work W and moving the converging lens 3 upward or downward while looking at a display of the focusing degree by the meter 34.
When the laser beam L is irradiated onto the work W, a portion thereof is reflected. When the laser beam is focused on the work W, the reflected beam becomes parallel to the original laser beam and returns through the transmission path that the laser beam came through as shown in FIG. 16B. On the contrary, in a case where the focal point is off from a right position, as shown in FIG. 16A and FIG. 16C, a portion of the reflected beam does not go back through the transmission path that the laser beam came through, and even if the portion of the reflected beam goes into the converging lens 3, it is scattered so that intensity of the reflected laser beam outputted from the second laser intensity sensor 32 becomes maximum when the focal point is focused at a right position.
Therefore, correlation between displacement from the focal point of the converging lens 3 and output from the divider 33 has the characteristics as shown in FIG. 17, and the output (the ratio c) from the divider 33 becomes maximum at the focus position.
Because of the features described above, when a value from the meter 34 becomes maximum by moving a position of the converging lens 3 upward or downward against the work W, a focus position of the converging lens 3 matches to a top surface of the work W.
In order to execute minute machining with high precision, it is necessary to minimize a diameter of the laser beam by narrowing down the laser beam at a position irradiated by the laser beam on the work W. Conventionally, measurement of the laser beam diameter at the position irradiated by the laser beam on the work W is executed by irradiating the laser beam onto acrylic resin or the like for machining to make a hole and measuring a diameter of the perforated hole with an appropriate measuring instrument.
In a case where machining with a laser beam is executed, generally a laser beam L is irradiated onto a work W, and at the same time an assist gas is injected in the same axial direction as that of the laser beam L for promoting combustion of a portion of the work to be machined or for suppressing dross formed on a rear surface of the work by blowing off melted material generated thereon, and for this reason, with a converging position maintaining apparatus based on the conventional technology as described above, if a work W is a extremely thin plate, the extremely thin plate is bent downward at a machining position from the standard position for irradiation of laser beam because of pressure of injected assist gas as shown in FIG. 14, which makes it unstable or even impossible for a contact 8 of a contact-type displacement meter 5 to follow the work W at the machining position.
As a result, a focus position does not exist on the extremely thin plate, and minute machining with high precision cannot be executed, and in an extreme case machining itself becomes impossible.
Also, with the converging position maintaining apparatus based on the conventional technology as mentioned above, it is necessary for the contact-type displacement meter 5 to be attached to or removed from the machining head 1 in a focus positioning operation, which makes the work complicated and requires a disadvantageously long period of time for the work.
A converged laser beam diameter is proportional to a cutting machining width, so that it is essential for executing minute machining with the laser beam to minutely converge the laser beam. FIG. 18 shows correlation (a computed value) between a displacement from a focus position and the converged laser beam diameter in a case where a YAG laser beam is converged by a lens with a focal distance of 50 mm. It can been seen from this figure that even a slight displacement largely changes a diameter of a converged laser beam.
However, focus positioning based on the conventional technology is executed according to a ratio of intensity of a reflected light from a surface of a flat-shaped work W with relatively loose change in intensity of a reflected light vs. an incoming beam thereto, so that it is difficult to execute focus positioning with high precision even in an operation for focus positioning while looking at a value of a meter 34.
In laser beam diameter measurement for measuring a diameter of a converged laser beam diameter by irradiating the laser beam onto acrylic resin or the like to make a hole and measuring a diameter of the perforated hole, only an intermittent measuring result can be obtained for a displacement from the focus position, and it is impossible to execute measurement with high precision. Also, use of materials such as acrylic resin and the like are rather wasteful.